During the previous funding period we made major advances in our understanding of HIV-1 uncoating and reverse transcription, publishing 36 manuscripts, including 19 with senior last authorship. Our salient finding was that cellular proteins and drugs that destabilize the HIV-1 core, thereby accelerating uncoating, prevent the occurrence of reverse transcription and disrupt infection. Overall our findings lead us to propose a model in which the HIV-1 core is a container that ensures the completion of reverse transcription before uncoating. Agents such as TRIM5? or the small molecule inhibitors PF74 and Bi2, which destabilize the HIV-1 core, prevent the occurrence of reverse transcription. Proteins and drugs that stabilize the HIV-1 core, however, also disrupt infection. To understand the mechanism by which stabilization of the HIV-1 core blocks HIV-1 infection, we are going to take advantage of the recently discovered human MxB protein. MxB is an interferon-?-inducible protein that blocks HIV-1 infection after reverse transcription but prior to integration. Genetic experiments suggested that capsid is the viral determinant for the ability of MxB to block HIV-1 infection; In agreement, our preliminary results indicated that MxB binds to the HIV-1 core and stabilizes the HIV-1 core during infection. MxB is the first naturally expressed protein known to stabilize the HIV-1 core, making MxB an ideal candidate to understand how core stabilization leads to a block on HIV-1 infection. This application will test the hypothesis that MxB binds to the HIV-1 core and forms higher order structures on its surface, thereby preventing uncoating. The following aims will be used to test this hypothesis: 1) Examine the role of MxB binding to capsid on restriction, 2) Examine the role of oligomerization and higher-order self-association in the ability of MxB to bind the HIV-1 core, 3) Examine the subcellular localization of restriction b MxB, and 4) Examine the role of phosphorylation in the ability of MxB to block HIV-1 infection. The knowledge gain by this proposal will be instrumental for understanding the basic uncoating process of HIV-1 and the mechanism by which MxB blocks infection. The knowledge gain here is the potential basis for the development of novel treatments against HIV-1.